Which is one of the two factors used to determine the heat stress index?

The heat stress index is a crucial tool used to assess the risk of heat-related illnesses in individuals, particularly in environments where physical exertion is prevalent, such as in firefighting. One of the key factors that contribute to the heat stress index is relative humidity. This is because relative humidity significantly affects the body's ability to cool itself through the evaporation of sweat.

When relative humidity is high, the air is saturated with moisture, making it harder for sweat to evaporate. This impairs the body's natural cooling mechanism, increasing the likelihood of overheating. The heat stress index incorporates relative humidity alongside temperature to provide a more comprehensive understanding of the heat stress risk. By doing so, it helps to inform individuals about their safety in high-heat environments and enables appropriate measures to prevent heat-related health issues.

Other factors, while relevant to thermoregulation, do not directly contribute to the computation of the heat stress index in the same manner.